A research program will be undertaken to study the role of histidine phosphorylation in eukaryotic cells. While protein phosphorylation is known to occur on several amino acids, it is fair to say that phosphoserine, phosphothreonine and phosphotyrosine capture most of the attention in the literature, at least as it pertains to cellular signaling in higher eukaryotes. This proposal turns the spotlight away from this group towards phosphohistidine (pHis). Phosphorylation of histidine is well recognized as being critical to signaling processes in prokaryotes and lower eukaryotes. However, pHis is becoming widely reported in mammalian signaling pathways and implicated in certain human disease states such as cancer and inflammation. Nonetheless, much remains to be understood about the role and extent of the modification in mammalian cell biology. Indeed, studying the functional role of pHis in signaling, either in vitro or in vivo, has proven devilishly hard, largely due to the instability of the modification. As a consequence, we are currently handicapped by a chronic lack of chemical and biochemical tools with which to study histidine phosphorylation. It is this problem that we seek to address in this research program. We have recently synthesized novel stable mimetics of pHis and used these to successfully raise the first antibodies against this modification in proteins. This breakthrough serves as the starting point for the proposed research program in which we will investigate the functional role of histidine phosphorylation on histone H4. It has long been known that histone H4 is phosphorylated on histidines 18 and 75. The precise role of this post-translational modification (PTM) is unknown, nor is the kinase that installs the PTM. A correlation exists between histone H4 histidine kinase activity and cell proliferation and, indeed, this activity is dramatically upregulated in certain cancer cells, specifically, in human hepato-carcinoma cells. We will use protein engineering methods in conjunction with a series of biochemical and biophysical approaches to test the hypothesis that histidine phosphorylation of H4 alters the compaction state of chromatin in a cell cycle dependent manner. The specific aims of this proposal are: (i) To develop a suite of antibodies against pHis epitopes with uses in the area of chromatin biology and beyond; (ii) To characterize the H4 histidine kinase and to identify H4 pHis binding factors, and; (iii) To elucidate the functional and structural role of H4 histidine phosphorylation in chromatin. This research program is designed to provide much needed research tools for studying pHis in cellular signaling. With these in hand, we will take the first steps towards elucidating the role of this PTM in chromatin biology. We expect this work to lay the foundation for understanding the role of this generally overlooked PTM in epigenetic regulatory mechanisms, a long term goal of this program.